Potential for Secondary Mineral Precipitation Down-Hole in Shale Gas Extraction Wells

Abstract

Hydraulic fracturing for shale gas production involves injection of pressurized water and additives into shale formations to enhance permeability. Interaction between injected fluid, formation water, and minerals in the shale has the potential to drive dissolution or precipitation reactions that will alter fluid composition and potentially formation porosity and permeability. The low initial permeability of shale plays makes it critical to minimize any processes that could further reduce permeability and gas production. We use a combination of field data and geochemical modeling to predict which reactions occur down boreholes during natural gas extraction. Hydraulic fracturing fluid (HFF) and flowback water compositions were monitored in three wells in southwestern Pennsylvania and these data are used to constrain reaction path models for subsurface interaction of HFF with host rock and formation water. Speciation and solubility calculations for HFF indicate that prior to injection the fluid is oversaturated only with respect to barite and iron oxides and hydroxides. Flowback water from the same well 7 weeks after initial injection is oversaturated with respect to multiple secondary minerals, including various sulfates (barite, jarosite), carbonates (aragonite, calcite, dolomite, strontianite), iron oxides and hydroxides (goethite, hematite, magnetite), and hydrated magnesium silicates (chrysotile, talc). The development of mineral oversaturation in flowback water may be due to either reaction with the host rock or formation water. Batch reaction modeling of HFF equilibrated with typical Marcellus shale minerals (illite, calcite, quartz, chlorite, smectite, pyrite) results in near saturation with respect to strontianite and oversaturation with respect to chrysotile and talc. Mixing models for HFF and formation water predict oversaturation with the same phases that are oversaturated in flowback water, plus additional carbonates (rhodochrosite, siderite, witherite), clays (chlorite, kaolinite, montmorillonite, smectite), and aluminum hydroxide (gibbsite). These geochemical models suggest mineral precipitation within shale gas boreholes could be of concern if reactions occur in the timeframe of gas production. Kinetic limitations and effects of organic compounds on precipitation should be considered, as they may determine the timescale and significance of dissolution and precipitation resulting from down-hole fluid mixing and water-rock interaction.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015